(1) Field of the Invention
The present invention relates to a system and method for controlling an ignition timing for an internal combustion engine. More specifically, the present invention relates to the system and method therefor, in which an engine knocking occurred in the engine is suppressed and simultaneously an MBT (Minimum Spark Advance For Best Torque) control is carried out so that an engine driveability is increased.
(2) Background of the Art
It is necessary to determine an ignition timing of an internal combustion engine according to an engine operating condition so that the engine is driven at optimum.
In general, it is a best way to ignite fuel supplied to the engine at a timing in a vicinity of a minimum advance angle while achieving a maximum engine torque, so called MBT, with an efficiency and fuel consumption of the engine taken into account. An MBT control such that the ignition timing is changed to MBT according to the engine operating condition has been carried out in almost engines.
However, an engine knocking occurs in a certain engine operating condition when an igniting timing advance angle is transferred to a more advance angle side and thereby a stable engine driving cannot be achieved.
A Japanese Patent Application First (Unexamined) Publication No. sho 58-82074 published on May 17, 1983 and U.S. Pat. No. 4,640,249 issued on Feb. 3, 1987 exemplifies the ignition timing controlling systems, in which a knocking control operation of controlling the ignition timing according to a presence or absence of the engine knocking is carried out in combination with the above-described MBT control operation.
In the ignition timing controlling systems disclosed in the above-identified Japanese Patent Application Publication and United States Patent, a pressure(s) in a combustion chamber(s) of an engine cylinder(s) (hereinafter referred to as an internal cylinder pressure) is detected and the ignition timing is controlled in the MBT control mode in such a way that a crank angle at which the internal cylinder pressure becomes maximum (hereinafter referred to as an internal cylinder pressure maximum timing) .theta..sub.pmax is placed at a predetermined angular position at which a magnitude of a generation torque of the engine becomes maximum.
In addition, at the same time, the engine knocking is detected by means of a signal processing circuit on the basis of the same detected signal indicative of the internal cylinder pressure. When the knocking level exceeds a predetermined value, the ignition timing is controlled toward a retardation angle side to avoid recurrence of the engine knocking (knocking suppressive control) with a higher priority than the MBT control. Furthermore, when the knocking has once been suppressed, the ignition timing is again controlled in the MBT control mode so that the magnitude of the engine generation torque becomes maximum. In this way, the magnitude of engine generation torque is made as large as possible along with the suppression of the engine knocking recurrence so that the engine driveability is improved.
However, in the ignition timing controlling system in the above-identified Japanese Patent Application Publication, whenever the knocking occurs, the knocking suppressive control is uniformly carried out with the higher priority than the MBT control. On the other hand, when the knocking does not occur, the MBT control causes the ignition timing to be corrected. When the knocking occurs, a correction angle quantity in the MBT control mode is fixed and the ignition timing is corrected to retard the ignition timing. Therefore, since in a case when the engine knocking has not occurred after the correction of the ignition timing toward the retardation angle side by means of the knocking control under some engine operating condition, the correction of the ignition timing toward an advance angle side is immediately executed by means of MBT control and thus an effect of avoiding the recurrence of the engine knocking is often lost.
To avoid such an situation as described above, if with the MBT control fixed for a predetermined interval of time after the occurrence of knocking, the engine operating condition abruptly changes, an angular point of the ignition timing angle in the MBT control mode is placed at the more retardation angle side. In this situation, a responsive characteristic of the MBT control is worsened and the effect of increasing the engine generation torque is reduced.